Method of operating piezoelectric transformers

ABSTRACT

In a high voltage generating device comprising a piezoelectric transformer having a pair of driving electrodes and an output electrode, and a driving source for applying a driving voltage across the driving electrodes, the frequency of the driving source is shifted a predetermined value about a frequency equal to n/2 times (where n is an integer) of the natural frequency of the mechanical vibration of the piezoelectric transformer.

United States Patent Kawada 1451 Sept. 12, 1972 [54] METHOD OF OPERATING 2,741,700 4/1956 Hall ..3l0/8.l UX PIEZOELECTRIC TRANSFORMERS 1,848,630 3/1932 l-lulburt ..3l0/8.2 3,047,823 7/1962 Ranky ..310/9.8 X [72] Invent 3 g 3,302,131 1/1967 Pyatt ..3l0/8.3 x p 3,432,691 3/1969 Shoh ..3l0/8.1 [73] Assignee: Denki Onkyo Company, Limited, 3,518,573 6/1970 Smith ..310/9.8 X

Tokyo, Japan 22] Fl d Se t 23 1970 FOREIGN PATENTS OR APPLICATIONS 880,557 10/1961 Great Britain ..310/s.2 [211 App]. No.: 74,549 I Primary Examiner-J. V. Truhe 30 F A Assistant Examiner-B. A. Reynolds 1 oreign "nation Priomy Dan Attorney-Chittick, Pfund, Birch, Samuels & Gauthier Sept. 29, 1969 Japan ..44/77675 [57] ABSTRACT [52] US. Cl. ..3l0/8.l, 3 10/82, 310/98 51 Int. Cl. ..H0lv 7/00 a genera?! i "P [58] Field of Seuch 310/8 1 8 2 8 3 8 7 9 5 piezoelectric transformer having a pair of driving elecb trodes and an output electrode, and a driving source for applying a driving voltage across the driving electrodes, the frequency of the driving source is shifted a [56] Reknnces Cited predetermined value about a frequency equal to 11/2 UNITED STATES PATENTS times (where n is an integer) of the natural frequency 3,598,909 8/1971 Sasaki ..310/9.s UX ifgfijg vibration of the piezoelectric trans' 2,596,460 5/1952 Arenberg ..3l0/8.l X 2,975,354 3/1961 Rosen ..3l0/8.l X 7 Claims, 9 Drawing Figures PATENTEDsEP 12 m2 SHEET 1 0F 3 a2 ww 5o SmSo FREQUENCY OF DRIVING SOURCE (Hz) R2 m6 bo .SQSQ

FREQUENCY OF DRIVING SOURCE (Hz) INVENTOR TAKEH I KO KAWADA ATTORNEY SHEET 3 0f 3 FIG. 8

, PATENTEDsim 1912 VOLUME PROPORTIONED TO THE MASS OF THE SOLDER (mm INVENTOR TAKE'HIKd KAWADA BY W? E ATTORNEY L A 3 l g 3 v w 5 a 3 m I\ m \GII m B l. a 6 9 v G [O F x a. .2 -LL 6 w 4 M Q fi Ivse Q2m30wmm m02 20mmm METHOD OF OPERATING PIEZOELECTRIC TRANSFORMERS BACKGROUND OF THE INVENTION former generally utilizes a piezoelectric material essenl tially consisting of barium titanate (BaTi0 or lead zirconate-titanate [Pb(ZrTi)0;,c]. The piezoelectric transducer comprises a substrate of this material, two driving electrodes applied on the opposite surfaces of one end of the substrate and an output electrode applied on the end surface of the opposite end. An AC driving voltage is applied across the driving electrodes to cause the substrate to vibrate at its natural frequency to produce a high voltage output at the output electrode.

The efficiency of the piezoelectric transducer is the highest when it is driven by a driving source having the same frequency as its own natural frequency. In other words, a maximum output voltage can be obtained with a minimum driving voltage. Under this condition, however, the percentage voltage regulation becomes extremely poor when the resistance load connected to the output electrode varies.

SUMMARY OF THE INVENTION The principal object of this invention is to provide a high voltage generating device utilizing a piezoelectric transformer which can operate stably.

Another object of this invention is to provide an improved high voltage generating device having small voltage regulation for the. varying load impedance of the piezoelectric transformer.

Still another object of this invention is to provide an improved high voltage generating device without sacrificing the voltage transformation ratio of the piezoelectric transformer.

Further object of this invention is to provide a high voltage generating device capable of operating at high efficiencies without increasing the internal loss of the piezoelectric transformer.

Another object of this invention is to provide a constant voltage high voltage generating device of small power wherein the temperature rise can be limited.

Yet another object of this invention is to provide an improved high voltage generating device wherein the resonance frequency of the piezoelectric transformer is readily variable.

Briefly stated, in accordance with this invention, the

' frequency F of the driving source for a piezoelectric transformer is shifted to the higher or lower level from a frequency which is equal to n/2 times (11 equals an integer) of the natural vibration frequency of the piezoelectric transducer by a suitable deviation frequency Af. When shifted in this manner, it is possible to improve the percentage voltage regulation of the piezoelectric transducer for the change in the load resistance. This prevents increase of the internal loss of the piezoelectric transducer and stabilizes the operating temperature. The deviation frequency is selected in a range not to substantially affect the voltage step-up ratio of the piezoelectric transformer.

The deviation frequency is obtained by varying the inductance of an inductive element or a coil of a series resonance circuit formed in the driving source for the piezoelectric transformer. Alternatively, a loading substance is mounted on the driving electrodes and or the output electrode of the piezoelectric transformer.

The frequency can also be shifted by varying the capacitance of a capacitive element on the output side of the piezoelectric transformer. By these means, it is possible to readily vary the deviation frequency Af.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a perspective view of a typical piezoelectric transformer utilized in this invention;

FIG. 2 shows an equivalent circuit of the piezoelectric transformer shown in FIG. 1 as viewed from the driving side;

FIG. 3 is a plot explaining the relationship between the frequency of the driving source and the output voltage for efiecting the required voltage step-up of the piezoelectric transformer;

FIG. 4 shows the characteristics showing the relationship between the frequency of the driving source and the output voltage when the mechanical Qm of the piezoelectric transducer is varied;

FIG. 5 shows the characteristics between the relationship between the frequency of the driving source of the piezoelectric transducer and the percent voltage regulation thereof;

FIG. 6 shows a connection diagram of one example of a high voltage generating device embodying this invention;

FIG. 7 shows characteristic curves to show the relationship between the frequency of the driving source, driving voltage and the percent voltage regulation actually obtained with the circuit shown in FIG. 6;

FIG. 8 shows a connection diagram of a television receiver employing the novel high voltage generating device and FIG. 9 shows characteristic curves showing the relationship between the volume of solder and the resonance frequency where the solder is used as a load for varying the resonance frequency of the piezoelectric transformer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference now to a typical piezoelectric transformer shown in FIG. 1, the piezoelectric transformer 10 comprises a substrate 11 of a piezoelectric material of the type referred to above, a pair of driving electrodes 12 and 13 applied on the opposite surfaces of one end of the substrate 11 and a high voltage output electrode 14 applied to the end surface at the opposite end. The natural frequency of the mechanical vibration fo of such a piezoelectric transformer is determined mainly dependent upon the length of the piezoelectric transformer 10. The relationship between the natural frequency f0 and the length L is expressed by an eq uation f0 & V/2L where V represents the velocity of the sound wave travelling through substrate 11 and L represents the length of the transformer 10. Also it is well known in the art that the voltage step-up ratio of the piezoelectric transformer is the maximum when the frequency F of a driving source, not shown, for applying an AC driving voltage across driving electrodes 12 and I3 is equal to the natural frequency fo of the piezoelectric transformer, and that when the frequency F of the driving source is selected to be equal to n/2 times of the natural frequency f of the piezoelectric transformer (where n is an integer) it is also possible to provide the maximum output voltage.

FIG. 2 shows an equivalent circuit of the piezoelectric transformer as viewed from the side of driving electrodes 12 and 13. The resonance frequencies of respective branch circuits are expressed by following equations The relationship between the output voltage of the piezoelectric transformer and the frequency of the driving source at resonance frequencies generally represented by (n/2)fo is shown by a plot of FIG. 3. FIG. 3 shows that very large output voltages can be obtained when the driving source frequency is selected to be equal to either lfo or f0.

Denoting the driving voltage impressed across driving electrodes 12 and 13 by V and the output voltage derived out from the output electrode 14 by V the voltage step-up ratio of the piezoelectric transformer 10 is expressed by the following equation:

At no load When a load RL is connected to the output side:

Where K The coefficient of electro-mechanical coupling in the direction normal to the direction of polarizatron,

K Thecoefficient of electro-mechanical coupling in the same direction as the polarization,

(1 zoss R An equivalent resistance of mechanical-loss of the transformer.

These equations show that the voltage step-up ratio of the piezoelectric transformer is proportional to the mechanical Om of the piezoelectric material and the length L of the substrate and reversely proportional to the thickness of the substrate. In other words, the longer the length, L, of the substrate and the thinner the thickness, the higher the output voltage V and the output voltage V increases with the mechanical Qm. When the mechanical Qm is varied, if the frequency and voltage V of the driving source were maintained constant the intervals of the characteristic curves are the same. For this reason, where the mechanical Qm is high, the frequency range in which the piezoelectric transformer undergoes voltage step-up function becomes narrower, whereas when the mechanical Qrn is low, the output voltage decreases and the frequency range in which the voltage step-up function is provided becomes wider. FIG. 4 shows these relationships in which curve a shows the output voltage for a high mechanical Qm and curve b that for a low mechanical Qm.

In this manner, the frequency range of the driving source which can be used to drive piezoelectric transformer 10 is limited. More particularly, when the mechanical Qm is high, the voltage step-up ratio is also high so that it is possible to provide a high output voltage with a relatively small driving voltage. However, when the frequency F of the driving source coincides with the resonance frequency (n/2)fo, the voltage regulation is poorest as above described. The same is true for a piezoelectric transformer of a low mechanical Qm.

FIG. 5 shows the relationship between the percentage voltage regulation p and the driving voltage, and the frequency F of the driving source, wherein curve a shows the driving voltage and curve b the percentage voltage regulation under a constant output voltage. As shown by curve a, the driving voltage is lowest at the resonance frequency (n/2)f0 and gradually increases on the opposite sides of this resonance frequency. Curve b shows that the percentage voltage regulation is the maximum for the resonance frequency (n/2)f0 and rapidly decreases or improved on the opposite sides of this resonance frequency. The driving voltage V near the points at which the percentage voltage regulation p is greatly improved is substantial equal to that at the resonance frequency n/Zfo. For this reason, if use is made of the frequency at this time, or the deviation frequency, it will be possible to decrease the percentage voltage regulation p without sacrificing the voltage step-up ratio so that it becomes possible to stably operate the piezoelectric transformer while maintaining the temperature rise due to the mechanical loss of the piezoelectric transformer at substantially a constant value. This is especially suitable for constant voltage high voltage generating devices of small power.

FIG. 6 shows a connection diagram of one embodiment of the novel high voltage generating device wherein like parts are designated by the same reference numerals as in FIG. 1. The driving electrodes 11 and 12 are connected across an AC driving source 20 having a frequency F which is variable by Afo about a frequency which is equal to n/2 times (n equals an integer) of the natural frequency of the mechanical vibration of the piezoelectric transformer. The high voltage output from the output electrode 13 is supplied to a load 40 through a voltage step-up rectifier circuit 30. A varia ble capacitor 41 is connected to the output electrode 13 for the purpose to be described later.

FIG. 7 shows a percentage voltage regulation characteristic (curve b) and a driving voltage characteristic (curve a) with respect to the frequency F of the driving source where the natural frequency (n/2)fo of the piezoelectric transformer 10 is selected near 62 KHz and the transformer is 56 mm in length. In this case, the power applied to load 40 is maintained constant. Curve a shows that when the frequency F of the driving source coincides with the resonance frequency (n/2)f of the piezoelectric transformer, the driving voltage V is the minimum (36 V). Under these conditions, the highest voltage stepup ratio and the highest power efficiency are possible. On the other hand, curve b shows that the percentage voltage regulation is the worst, that is from about 70 to 100 percent. Thus, the output voltage varies greatly for load variations under this condition, so that the operation of the piezoelectric transducer is unstable. As the driving frequency is shifted 100 to 200 Hz to the lower side of the resonance frequency (n/2)f0 of the piezoelectric transformer, the percentage voltage regulation is greatly improved and when the driving frequency is shifted more than 200 Hz, the percentage voltage regulation will assume a flat curve of about to percent.

Conversely when the source frequency F is shifted to the side higher-than the natural frequency (n/2)f0, only when a shift of 700 to 800 Hz is given, significant improvement of the percentage voltage regulation p is attained and then the value of p decreases gradually. At a shift of more than 800 Hz, the percentage voltage regulation assumes a nearly flat curve of about 15 to percent. Thus in order to operate the piezoelectric transformer under stable and improved condition of percentage voltage regulation for varying load it is necessary to shift the driving frequency about 200 to 500 Hz on the lower side while about 800 to 1,100 Hz on the higher side with respect to the resonance frequency (n/2 )fo.

As above describe, the position at which the percentage voltage regulation is improved rapidly or the deviation frequency Afo varies dependent upon the value of the frequency F of the driving source, that is, values of n and f0 of the resonance frequency (n/2)fo of the piezoelectric transformer but the deviation frequency tends to increase in proportion to frequency F.

FIG. 8 shows a connection diagram of a television receiver employing a high voltage generating device driven by the method of this invention, in which similar parts as in FIG. 6 are designated by the same reference numerals. The television receiving circuit comprises a horizontal deflection circuit 50 also comprising a portion of the driving source 20 and a series resonance circuit 60 including a capacitor 61 and a variable inductor 62, said resonance circuit 60 being driven by the pulse voltage generated by the horizontal deflection circuit 50 whereby to impress across the driving electrodes of the piezoelectric transformer driving signals having a frequency shifted from the frequency of (n/2)f0 by a deviation frequency Afo. The horizontal deflection circuit 50 comprises a transformer 51 supplied with a horizontal synchronizing signal, a deflection coil 52, a damper diode 53, a flyback transformer 54, a horizontal output transistor 55, and capacitors 56, 57 and 58. The secondary winding of the flyback transformer 54 provides video signals through a rectifier circuit 70 comprised by a diode 71 and a capacitor 72. e

To the output electrode of the piezoelectric transformer is connected a voltage step-up rectifier circuit 30 comprising diodes 31, 32, 33 and 34 and capacitors 35, 36 and 37 which are connected to provide a step-up ratio of 4. The output of the voltage step-up rectifier circuit 30 is connected to the anode electrode (not shown) of a cathode ray tube 40, that is the load.

The frequency F of the driving source 20 is determined such that it is equal to n/2 times of the resonance frequency f0 of the piezoelectric transformer by the proper adjustment of the variable inductor 62 of the series resonance circuit 60. In one television system, the output electrode of the horizontal output transistor 55 provides a horizontal synchronizing signal or a pulse voltage of a frequency of 15,734 Hz which is converted into a sinusoidal wave by the action of the series resonance circuit 60. As above described stable operation of the piezoelectric transformer can be assured when the deviation frequency Afo from the resonance frequency (n/2)f0 is selected to be in a range of from about 200 to 500 Hz. Generally, the most suitable value of the load of the piezoelectric transformer ranges from 25 to 50 megohms in order to provide efficient power conversion. However, since the anode resistance of the cathode ray tube is generally more than megohms it is usual to connect to the output side of the piezoelectric transformer a step-up rectifier having a step-up ratio of 4 or 6 to reduce heating of the piezoelectric transformer thus assuring efficient and stable operation thereof.

Instead of varying the variably inductor 62 it is also possible to vary capacitor 61.

Another method of varying the frequency of the source around a frequency which is equal to n/2 times of the natural frequency f0 of the piezoelectric transformer involves securing a loading substance such as solder and the like to either driving electrodes 12, 13 or output electrode 14 or both of driving and output electrodes thus varying the resonance frequency of the piezoelectric transformer. The position of attaching such a loading substance and the mass thereof are determined by taking into consideration the vibration mode and the resonance frequency of the piezoelectric transformer. Thus, for example where it is desired to decrease the resonance frequency by a certain amount the loading substance may be secured to one ends of the driving electrodes.

FIG. 9 shows the resonance frequency characteristics of a piezoelectric transformer where solder is utilized as the loading substance, wherein the ordinate represents the resonance frequency fo (in KHz) of the piezoelectric transformer and the abscissa the volume proportional to the mass of the solder. Curve 0 shows the resonance frequency characteristic with the solder secured to the driving electrodes whereas curve b that with the solder secured to the output electrode. These curves show that the variation in the resonance frequency is more significant when solder is secured to the output electrode 14 than the case wherein solder of the same mass is secured to the driving electrodes 12 and 13. More in detail, where solders of the varying volume of from 2 to 14 mm are secured to the output electrode 14 the resonance frequency varies about 3 KHz, whereas when solders of the varying volume of from 3 to 20 mm are secured to the driving electrodes 12 and 13 the natural frequency varies only about 2 KHz.

It is to be noted that, however, in any case, it is possible to readily vary the resonance frequency over a considerably wide range.

Alternatively, a variable capacitor may be connected to the output side of a piezoelectric transformer for the purpose of varying the natural frequency thereof. Such a capacitor is shown in FIG. 6 as at 41.

Although the invention has been shown and described in terms of some preferred embodiments thereof, it will be clear that many changes and modifications may be made without departing from the true spirit and scope of the invention as defined in the appended claims.

What is claimed is:

l. in a high voltage generating device comprising a piezoelectric transformer having a pair of driving electrodes and an output electrode, and a driving source for applying a driving voltage across said driving electrodes, a method of operating said high voltage generating device which comprises the steps of shifting the frequency of said driving source by a predeterming deviation frequency from a frequency equal to n/2 times (where n is an integer) the natural frequency fo of the mechanical vibration of said piezoelectric transformer and driving said transformer with said shifted frequency.

2. The method according to claim 1 wherein the frequency of said driving source is shifted to the higher side of said frequency of (n/2)f0.

3. The method according to claim 1 wherein the frequency of said driving source is shifted to the lower side of said frequency of (rt/2 )fo.

4. The method according to claim 1 wherein the frequency of said driving source is varied by connecting a series resonance circuit including a capacitive element and an inductive element to said driving source and by varying said inductance element.

5. The method according to claim 1 wherein the frequency of said driving source is varied by connecting a series resonance circuit including a capacitive element and an inductive element to said driving source and by varying said capacitance element.

6. The method according to claim 1 wherein the frequency of said driving source is varied by connecting a capacitive element between said driving source and said driving electrodes of said transformer and by connecting an inductive element between said capacitive element and ground and by varying said inductance element.

7. The method according to claim 1 wherein the frequency of said driving source is varied by connecting a capacitive element between said driving source and said driving electrodes of said transformer and by connecting an inductive element between said capacitive element and ground and by varying said capacitance element. 

1. In a high voltage generating device comprising a piezoelectric transformer having a pair of driving electrodes and an output electrode, and a driving source for applying a driving voltage across said driving electrodes, a method of operating said high voltage generating device which comprises the steps of shifting the frequency of said driving source by a predeterming deviation frequency from a frequency equal to n/2 times (where n is an integer) the natural frequency fo of the mechanical vibration of said piezoelectric transformer and driving said transformer with said shifted frequency.
 2. The method according to claim 1 wherein the frequency of said driving source is shifted to the higher side of said frequency of (n/2)fo.
 3. The method according to claim 1 wherein the frequency of said driving source is shifted to the lower side of said frequency of (n/2)fo.
 4. The method according to claim 1 wherein the frequency of said driving source is varied by connecting a series resonance circuit including a capacitive element and an inductive element to said driving source and by varying said inductance element.
 5. The method according to claim 1 wherein the frequency of said driving source is varied by connecting a series resonance circuit including a capacitive element and an inductive element to said driving source and by varying said capacitance element.
 6. The method according to claim 1 wherein the frequency of said driving source is varied by connecting a capacitive element between said driving source and said driving electrodes of said transformer and by connecting an inductive element between said capacitive element and ground and by varying said inductance element.
 7. The method according to claim 1 wherein the frequency of said driving source is varied by connecting a capacitive element between said driving source and said driving electrodes of said transformer and by connecting an inductive element between said capacitive element and ground and by varying said capacitance element. 